M. Dorlochter et al., DIFFERENTIAL CALCIUM SENSITIVITY OF 2 TYPES OF QUANTUM BUMPS IN LIMULUS VENTRAL NERVE PHOTORECEPTORS, Journal of comparative physiology. A, Sensory, neural, and behavioral physiology, 178(6), 1996, pp. 863-869
Ventral photoreceptors in Limulus polyphemus were continuously illumin
ated with dim light and produced two types of quantum bumps. Bumps of
one type had a relatively symmetrical shape and fast rise and decay ki
netics. In addition, bumps were detected with a small amplitude and sl
ower kinetics. We termed these bump types ''C-2'' and ''C-1 bumps'', r
espectively. The half width of a bump divided by its amplitude was fou
nd to be a reliable criterion to distinguish between the bump types. T
his shape quotient is smaller than 0.5 ms/pA for the C-2 bumps and lar
ger than 1 ms/pA for the C-1 bumps. Lowering the extracellular calcium
concentration from 10 mM to 0.25 mM caused an increase in the average
amplitude of C-2 bumps to 196%. After the injection of small amounts
of 1.2 mM EGTA solution the amplitude of these bumps was reduced to ab
out 30 and 50% in two cells studied. By contrast, in both cases C-1 bu
mp amplitudes were not affected significantly. We conclude that the tw
o bump types are generated in one photoreceptor by the activation of d
ifferent transduction pathways. These pathways are differentially sens
itive to changes of the cytosolic calcium concentration.